This research focuses on the discovery of genetic and developmental mechanisms that govern evolutionary change in embryogenesis and early larval development. By using a non-model system with a heritable developmental dimorphism, I can directly ascertain key mechanistic changes in early development and oogenesis within a single species. This approach eliminates the potentially confounding effects of more traditional comparative approaches, and has applications for genetic diagnosis of fertilization competent eggs. Here I propose to use the developmental dimorphism in the marine polychaete Streblospio benedicti to characterize the extent of genomic divergence contributing to egg size and larval development. Using quantitative genomics and comparative transcriptomics, I will determine the specific loci involved in regulating this development mode determination, and quantify the extent that these variants act maternally, zygotically or both. Specifically, I will ue linkage mapping to identify quantitative trait loci (QTLs) associated with developmental and reproductive traits. In addition, I will use the transcriptomes of oocytes and early embryos to ascertain how these two offspring types differ in abundance and timing of gene expression. The proposed research is the first study to investigate how critical developmental determinants such as egg size can evolve using a single species.